Tanning compositions



I. Patented Feb. 19, 1946 TANNING COMPOSITIONS Joseph S. Kirk, Seven Hills, 01110, assignor to E. L du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware No Drawing.

Application February 25, 1942, Serial No. 432,268

This inventionrelatesto tanning. and is more particularly directed to dry tanning compositions comprising an acid-soluble silicate, an acid-soluble aluminum compound, and a masking agent, the'dry compositions being adapted to dissolution in aqueous media at a low pH to give low molecular weight silicic acid, is further directed to tanning solutions comprising low molecular weight silicic acid, an acid-soluble aluminum compound, a masking agent, and sumcient acid to give a pH less 'than about 3.5, and is still further directed to tanning processes comprising subjecting a skin to the action of an aqueous solution comprising low molecular weight silicic acid, an acid-soluble aluminum compound, and a masking agent, at a pH of about from 1.0 to 3.5'and raising the pH of the solution while the skin is in contact therewith.

Various effortshave been made to, tan skins with compositions containing silicates or silicic acid. The leather obtained, however, has been of definitely inferior grade, being brittle, harsh,

' that is, overtanned surface layers and undertanned inner layers in the skin, which has been a serious disadvantage with silicic acid tannages heretofore available is completely obviated.

In silicic acid tanning solutions hitherto proposed the silicic acid content has been kept relatively low, that is, around one-half per cent or less .by weight, and consequently tanning with such solutions has been unduly slow, laborious, and expensive. Theprocesses and compositions of the present invention make practicable the use of much higher silicic acid concentrations, even in the presence of aluminum compounds and thus permit more rapid and inexpensive tannages using these compounds.

Recently tanning processes have been described in which leather of greatly improved quality is produced by the conjoint use of silicic acid with a hydrogen bonding agent. In such baths an aluminum salt may also be used. According to of excellent quality is produced without employing a hydrogen bonding agent although in a preferred aspect of the invention a hydrogen bonding agent may also be used.

It is an object of this invention to provide tanning compositions and processes giving leather of excellent quality. Another object is to provide prepared compositions which can be dissolved in an acidulous medium to produce tanning compositions comprising silicic acid sols and suitable aluminum compounds together with masking agents. compositions which may be used in tanning processes with a minimum of chemical control by the tanner. Another object is to provide tanning .compositions and processes operative at such a pH range that rapid tannage of skins is obtained without astringency or case hardening and with a minimum of loss due to precipitation of tanning materials not in combination with the skins. Other objects of the invention will appear hereinafter. I

The foregoing and other objects are accomplished according to this invention by providing,

for use intanning, dry compositions adapted to the processes of the present invention, leather dissolution in aqueous media at low pH to give low molecular weight silicic acid, the compositions comprising. an acid-soluble silicate, an acidsoluble aluminum compound, and a masking agent. The objects are further accomplished by providing tanning solutions in which low molecular weight silicic acid, an acid-soluble aluminum compound, and a masking agent are present at a pH below about 3.5, and are still further accomplished by tanning processes in which a skin is subjected to the action of such a tanning solution at a pH of about 1.0 to 3.5, and the pH of the solution is raised while the skin is in contact therewith.

As the source of silicic acid in the dry tanning compositions there may be used any silicate or silica compound which does not contain constituents deleterious to the tanning bath and which is capable of dissolving in acid at a pH below about 3.5 to give low molecular weight I silicic acid. Similarly, as the source of dissolved aluminum there may be used any aluminum compound capable of dissolving in acid at a pH 'below about 3.5. Preferred compositions for use in this invention may be offered 'to the trade in the form of dry mixtures containing a source of both silicic acid and soluble aluminum and a masking agent in proper proportions. Thus, there may be furnished to the tanner a dry mixture of sodium metasilicate, basic aluminum sul- Another object is to provide prepared fate, and sodium acetate from which a tanning solution may be made simply by adding the dry mixture to acid while maintaining the pH at a suitably low figure, Ideally, the silicic acid and soluble aluminum may be furnished as a single compound, such as acid-soluble sodium aluminum silicate, and the masking agent may be admixed with the silica-alumina compound. Optionally, a hydrogen bonding agent may be included in the compositions as furnished to the tanner.

In making up a tanning solution according to this invention 'a tanner 'will ordinarily prefer to use dry, prepared compositions as just described, and will thereby secure such advantages as exact proportioning of constituents and confinement of necessary chemical control to a minimum. It will be understood, however, that solutions of silicic acid may be made by a variety of methods already known to the art, and that any such method may be employed in preparing Y silicic acid for a tanning solution of this invention provided it is capable of giving a silicic acid of suitably low molecular weight at this specified pH. Typical of methods of preparation which may be used are the following.

From methyl silicate.-Method described by Grimaux, Compt. rend. 98, 105 (1884); 98, 1434 (1884). According to my modification of this method, sols may be prepared by stirring methyl silicate with a sufficient quantity of water so that there are say parts of SlOz per 100 parts of the mixture. The addition of sufficient acid to lower the pH to about 1 or 2 accelerates the rate of hydrolysis of the methyl silicate and increases the stability .of the silicic acid in a low molecular weight state.

From silicon' tetrachloride.--E. C. Williams, U. S. Patent 1,539,342. Silicon tetrachloride is stirred into Water with agitation and in such proportion as to give a solution of silicic acid which gels on standing. One method of carrying out this reaction would be to run silicon tetrachloride in a thin strea minto a slightly acidi fied mixture of water and ice until, for example, the mixture contains 4 per cent by weight of SiO'z. The excess acidity is then neutralized with alkali such as 20 per cent NaOI-I solution with violent stirring as the alkali is added, until a pH of 1 or 2 is reached.

From silicon sulfide.1=leury, U. S. Patent 61,931. By stirring silicon sulfide with water, a solution of silicic acid can be obtained with the simultaneous evolution of hydrogen sulfide.

By electro osmosis of sodium silicate solution.- Schwerin, U. S. Patent 1,132,394. A dilute solution of sodium silicate is subjected to electroosmosis, the silicic acid being obtained as a solution in the anode compartment.

By the electrolysis of sodium silicate with a mercury cathode.N. L. Collins, U. S. Patent 1,562,940. By electrolyzing a solution of sodium silicate in a cell fitted with a lead anode and mercury cathode, there is obtained a solution of silicic acid. By adding a small quantity of salt, such as sodium chloride, to the solution just before hydrolysis, the stability of the silicic acid would be increased by virtue of the small amount of hydrochloric acid thereby formed at the anode.

While the above-described methods of preparation may be used in making silicic acid for a tanning solution of this invention distinct advantages are realized when the silicic acid is made by dissolving dry, prepared tanning compositions in aqueous media at a pH below about 3.5, as already mentioned. Even greater advantages accrue when the silicic acid and aluminum are both part of a single compound, as is the case with sodium aluminum silicate. With such a single compound the danger of non-uniformity of mixing of dry components is eliminated, fluctuations of the pH during dissolution of the aluminum and silicic acid constituents are minimized, and the proportion of active ingredients in the dry tanning composition is usually the maximum obtainable.

Sodium aluminum silicates for use in the prepared tanning compositions of this invention suitably may be naturally occurring products, such as for instance, analcite, naturolite, or nethelite. Natural sodium aluminum silicates will frequently contain insoluble materials and will frequently be made up of mixtures of silicates, but these are all suitable so long as they form silicic acid sols admixed with a dissolved aluminum compound upon being dissolved in a dilute acid.

Instead of these naturally occurring silicates there maybe used any of the similar compounds prepared commercially. Typical of these compounds are sodium aluminum silicates, prepared, for instance, by reacting a silicate with an aluminum compound. Examples of such materials will be found in the Benson Patent 2,244,383 and in the Schorger Patent 1,949,360. Sodium aluminum silicates are also produced by heating kaolin with a sodium hydroxide solution. While such naturally occurring or synthetically prepared sodium aluminum silicates may be used, the preferred practice of the present invention is to use a water-insoluble, acid-soluble, precipitated alkali-metal aluminum silicate, such as sodium or potassium aluminum silicate, which is made at a high pH by processes more particularly described hereinafter.

Aluminum silicate compositionsused ought to have a mol ratio of AlzOazSiOz of 1 :Zto 1 :4. Natural sodium aluminum silicates selected or artificial materials prepared ought to have an AlzOsZSlOz ratio within, the limits. It will be understood. however, that if a natural or prepared sodium aluminum silicate has a ratio different from that which is desired, this can be accomplished at least to some extent by adding an aluminum compound such as alum to increase the A1203 content. In the case of prepared sodium aluminum silicate, it will ordinarily be desirable to predetermine the ratio, and mixtures may be made with natural materials to produce a pi'oduct having substantially any desired ratio for tanning.

The sodium aluminum silicates which it is preferred to use accordingto this invention are precipitated products which are acid-soluble, amorphous, soft, chalky, white, and opaque. The ratio AlzOalSiOz may vary from about 1:2 to 1 :5 though the preferred product has a ratio between about 1:3 to 1:4. The precipitated sodium aluminum silicates are made at a high pH and contain less than 1.5 mols of Na20 per mol of A1203.

In the preparation of such precipitated silicates it will ordinarily be found desirable to use sodium silicate for the reaction, but it will be understood that any silicate more soluble than its products of reaction with the aluminum compound may be used. There may be used, for instance, potassium or lithium silicates. The SiOzzNazO ratio ought not to be substantially greater than 2. It is preferred to use sodium metasilicate, but if a silicate with a higher SiOmNazO ratio is used, an alkali will need to be added. In any event, the total alkali in the reaction should be suflicient to maintain a pH of 9 or more.

If an alkali is required, then any alkali may be used which will bring the pH of the reaction to at least 9, tho of course the 'alkali should not be one which will react with the silicate or aluminum compound to form insolubleby-products.

As examples of suitable alkalies there may be easier to maintain a pH of 9 or more during the reaction.

It will be found a comparatively simplmatter to adjust the composition of the precipitated sodium aluminum silicate since almost all, say over 90 per cent, of the silica and alumina used in the reaction will be found in the product. The proportions of reactants should be soselected that the AlzOaISiOz ratio will fall within the limits from about 1:2 to 1:5. Preferably the product will have an AlzOatSiOz ratio of from about 1:3 to 1:4. Products within these ratios will be found soluble in a 0.1 normal hydrochloric acid solution.

The alkalinity of the product should be kept down, and the product should contain not more than 1.5 mols of NazO per mol of A1203. The use of large excesses of alkali in the reaction should therefore be avoided, and the product should be carefully washed.

To effect reaction the soluble silicate and the soluble aluminum compound are each first put in solution and then these solutions are brought together". .The solution of silicate may ordinarily contain from about 5 to 'per cent of solids and the solution of the aluminum compound may similarly contain about 5 to 15 per cent solids. The final mixture should not contain much in excess of about to per cent solids by weight.

It will be understood that in effecting reaction it will ordinarily be found desirable to add the aluminum compound solution to the silicate solution. In this way one may most easily avoid the presence of local concentrations of aluminumcompound having a pH below 9. It will be found possible, however, to run the two solutions simultaneously into a mixer in proper ratio, and in general any method of mixing may be effected so long as low pH is avoided.

As has been indicated, a pH substantially below 9 should not be permitted for a substantial period of time during the reaction, but if the reaction is conducted at a pH of 9 or above, then the pH may be reduced toward the end of the reaction period to as low as 7 or 8 without markedly injuring the product. It may frequently be found desirable after the reaction is substantially concluded, therefore, to reduce the pH so that the product will b substantially neutral.

After the reaction is concluded the precipitate may be filtered and washed. The product may be made more easily filterable by effecting the reaction at superatmospheric pressure or at elevated temperatures or both. Ordinarily, however, no substantial beneflt is derived from the use of either heat or pressure.

The masking agents used with siliconaluminum compositions according to this inven- When an aluminum compound such as sodium aluminate is used, it will of course be There may be used, for instance,

tion may be any of the materialsheretofore used and recognized as being effective in the allied art of chrome tanning. Thus, there may be used, for instance, lactates, acetates. propionates, tartrates, glycollates, citrates, oxalates, adlpates, succinates or glutarates of sodium, potassium, ammonium, or an alkaline earth metal, the cation being unimportant provided it is compatible with the other constituents of the tanning bath in which it is to be used. Instead of salts, free organic acids maybe used. Preferably the masking agent should be an organic carboxylic acid having not more than 8 carbon atoms or a salt of such an acid.

The amount of masking agent to use may be considerablyvaried depending on the conditions employed for the tanning, but in general it will be preferred to use small amounts in accordance with practices customary inthe chrome tanning art since too much of the masking agent will too greatly retard tanning. In general from about 5 to 500 per cent of the weight of alumina calculated as A1203 may be used. Of the available masking agents acetic acid, lactic acid, and glycollic acid or their salts are preferred, and particularly desirable results have been obtained by mploying a composition comprising precipitated sodium aluminum silicate and sodium acetate and by dissolving this dry composition in acid to make a tanning bath.

The desirable results obtained according to this invention may be further enhanced by the inclusion in the tanning composition of a hydrogen bonding donor compound. Hydrogen bonding is a concept advanced in recent years to explain certain abnormalities in the chemical and physical behavior of mixtures of compounds, one of which contains hydrogen attached to a strongly negative radical and the other an atom capable of donating a pair of electrons to form a directional or co-ordination bond. Since the bond is ,formed by the donation of an electron pair from one atom, the donor, to the other atom, the bond is not of the type-conceived as an ordinary 7 valence bond but many of the properties of the mixture indicate that a type of chemical compound is formed. These mixtures, for instance, exhibit an abnormal vapor pressure lowering, that is, a deviation of Raoults law. There is further observed abnormal heats of mixing and abnormal viscosities and freezing point lowerings.

When silicic acid is mixed with an organic hydrogen bonding donor compound, it exhibits characteristics unexplainable upon the basis of ordinary chemical reaction and unexpected from a consideration of the characteristics of the materials mixed. Apparently silicic acid has an acceptor hydrogen atom and forms some type of compound with a hydrogen bonding donor. Among the characteristics of these silicic acidhydrogen bonding donor combinations is a de-.

creased tendency to precipitate gelatin and an increase in the time required for the sol to be converted to a gel.

It will be understood that in advancing the theory of hydrogen bonding it is not intended to limit or restrict the present invention, the explanation being given only for purposes of i1- lustrationf Whatever the reason, the type of compounds known to be organic hydrogen bonding: donors profoundly modify silicic acid sols and may be included in the compositions of the present invention.

It will be seen that hydrogen bonding donor compounds are of the type of materials which might be designated as weak organic bases. According to this theory, the materials involved act as bases with silicic acid which is quite a strong acid. These bases are of such weakness that they will not raise the pH of a silicic acid solution above 7. According to this theory one may therefore describe the agents added to silicic acid sols according to the present invention as water-soluble, organic bases which will not raise the pH of silicic acid above 7. It is again observed that. while the terminology of either the hydrogen bonding" theory or the weak base" theory may advantageously be used indescribing the present invention to those skilled in the art in commonly understood language, applicant does not consider the adoption of either theory as bearing upon the invention save as it gives a convenient system of terminology.

While any hydrogen bonding donor compound (or weak base) may be used with silicic acid and an aluminum compound according to the present invention, it will be generally found desirable to use any water-soluble hydrogen bonding donor compound selected from the group consisting of ethers, amides, alcohols, ketones, and esters of phosphoric acid.

Hydrogen bonding donor compounds used in the compositions of this invention should be water-soluble at least to the slight extent required to permit formation of a coordination complex of silicic acid. In additon to being water soluble, a

hydrogen bonding donor should not contain groups which will react with silicic acid in aqueous acidic media to give compounds of conventional sort when such reaction would interfere with hydrogen bonding. The donor preferably should not react with silicic acid to give a precipitate.

As has been indicated, the water-soluble organic hydrogen bonding donor compounds used according to this invention are preferably selected from the group consisting of ethers, amides, alcohols, ketones, and esters of phosphoric acid. It is preferred that the ethers have an oxygen or nitrogen atom in addition to that in the ether linkage, and specifically it is desirable that the ether containin addition to the other group a second ether group, a hydroxy group, an amide group, or an ester group. In the ethers, the two carbon atoms attached to the oxygen atom of the ether linkage should not be attached to each other. Amides for use with silicic acid sols according to the present invention are preferably N-substituted. Alcohols for use as donors ought to contain more than two carbon atoms. A1- cohols are among the weakest of the hydrogen bonding agents but may find value in some instances. Ketones are not quite as weak as alcohols and any water-soluble ketones may be used as above discussed. Esters of phosphoric acid are quite effective hydrogen bonding agents, and may frequently be included in compositions of this invention to advantage.

Ethers are among the preferred hydrogen bonding donor compounds for inclusion in compositions according to the present invention. In addition to containing an ether group these donors may advantageously contain an additional ether group, a hydroxy group, an amide group, or an ester group. The presence of these groups ap- A number of such groups pears very beneficial. may be present and there may be used, forinstance, polyethers which contain hydroxyl groups and ester groups. lowing are listed:

Nonaethylene glycol Dimethyl ether of tetraethylene glycol Dimethyl ether of diethylene glycol Hexaethylene glycol Butyl ether of diethylene glycol Ethyl ether of diethylene glycol acetate Methyl ether of diethylene glycol acetate Tetraethylene glycol Monoethyl ether of diethylene glycol Triethylene glycol Monoethyl ether of ethylene glycol Monomethyl ether of ethylene glycol Diethylene glycol N,N'-Bis(beta-methoxyethyl) adipamide N,N'-Bis(methoxymethyl) urea Polyethylene glycol adipate Diethyl ether of diethylene glycol Dioxane Dioxolane Diethyl ether of ethylene glycol Dimethyl ether of ethylene glycol Triethylene glycol dipropionate Glycerol formal Ethylene g ycol acetal Sorbitol triacetal N,N-Dimethylmethoxyacetamide N-Acetylmorpholine N,N'-Adipyldimorpholine Dimorpholide urea Polyethylene oxide Monomethyl ether of ethylene glycol-ethylene oxide reaction product Ethylene glycol-ethylene oxide reaction prod 'uct Glycerol-ethylene oxide reaction product Ethanolformamide-ethylene oxide reaction product As examples of ethers the fol- A-mides are among the preferred hydrogen bonding donors for use with silicic acid sols according to this invention. Whereas oxygen is the donor atom in ethers the nitrogen of amides probably acts as the donor atom. Among the most effective compounds of this group are the N-substituted amides and the di-substituted compounds are preferred. It will be seen that some of the compounds in the above list of ethers are amides as well and could have been shown below as illustrative of the use of amides.

Examples of amides are listed below, ureas and other amides being listed separately:

Urea

Tetramethylurea Tetraethylurea Urea 'Ihiourea N,N'-Diethyl-N,N-diethanolurea Amides:-

N,N,N,N'-tetramethyladipamide N,N-dimethylacetamide N,N,N',N"-tetramethylsuccinamide N,N,N,N'-tetraethylsuccinamide N,N-diethylacetamide N,N,N',N-tetraethyloxam.ide N,N-diethylformamide N,N-diethylpropionamide N,N-dimethylformamide N,N,N',N'-tetraethyloxamide N,N-diethylglycolamide N-ethyl-N-ethanolformamide N-ethyl-N-ethanolglycolamlde- Examples of ketones may be Ketones:

Acetonylacetone Acetone Formacetoethyl Methyl acetoacetate' Diacetone alcohol Examples of alcohols may be given as follows: Alcohols:

Diacetone alcohol 2-methyl-2,4-pentanedio1 Isopropanol Pinacol Hexamethylene glycol Pentaglycol Tert-butyl alcohol Tetrahydroxy octane 1,1,1-trimethylolethane Propanediol N-butanol iven as follows:

Esters of phosphoric acid:

Tributyl phosphate Triethyl phosphate Tri-isopropyl phosphate Tri-isoamyl phosphate Hexyl phosphate I Tri(butyl ether of diethylene glycol) phosphate The amount of hydrogen bonder to use in a particular composition varies considerably with the results desired and the particular tanning procedure to be followed. The amount of donor should, of course, not be so greatas to slow the tanning unduly. The precise amount of a particular agent to use in a particular instance can best be determined by making up a few sample compositions and trying tanning baths made with them. In general it may be indicated that when a donor is used the amount should be such that when a tanning solution has been made up with the composition the donor will-be equivalent to from about 0.1 per cent to about per cent of the weight of the tanning solution.

Compositions according to the present invention may contain still other modifying agents and there may be included, for instance, other tanning agents and tanning assistants.

In making up tanning solutions with a composition of this invention it is only necessary to dissolve the composition in a dilute acid, Ordinarily, dilute sulfuric acid sufiicient to bring the pH to a proper value for tanning will be used. Altematively'there may be employed such acidic materials as sulfamic acid, hydrochloric acid, nitric acid, thionic acids, acetic acid, lactic acid, and other similar inorganic and organic acids. It acids having masking action, such as acetic and lactic, are used, it will be necessary to use also a stronger acid since otherwise the amount of acid having masking action will be greater than is desirable in the tanning bath. Acidulous salts may similarly be used, and there may be employed, for instance, sodium bisulfate, monosodium phosphate, acid tartrate, zinc chloride,

titanium sulfate, aluminum sulfate, and chromium sulfate. In short, any acidic substance may be used to reduce the pH provided the substance selected does not interfere with the action of the bath. The amount of acid or acidic material will ordinarily be such as v to resultin a pH of about 10 to 3.5.

It will ordinarily be found desirable'to use such an amount of acid as will give a taming bath having a pH of about 1.0 to 3.5 after the siliconand aluminum-containing constituents have been added. Stillmore specifically it will be preferable to-reduce the pH to about 2 to 3. It will be understood that while any acidulous material may be used to increase the acidit of silicate solution to produce silica sols, some acidulous materials do not themselves have. a low enough pH to produce the final acidity desired. In this event such an acidulous material may be used in combination with a more acidulous material,

I needs of particular specialized problems.

and as will be evident mixtures of two or more acidulous materials may be used as seems desirable.

Acids may be included in compositions of the present invention by mixing the compositions with such dry acids and acidulous materials as sulfamic acid and sodium acid sulfate. To minimize the reaction between the solid acid and the silicate employed, one or both of the ingredients can be coated with an inert prior to mixing If the compositions are to be stored any length of time there is unfortunately a tendency to premature reaction and it will ordinaril be found desirable under these circumstances to offer the silicon-containing components and the acid-containing components separately, or else simply as indicated above, to let the tanner add the composition to a dilute acid solution.

The concentration of tanning agent in tanning solutions made up using compositions of the present invention may be considerably varied. The concentration will depend specifically upon numerous factors in the tanning process itself. 0rdinarily, an S102 content of about 6 per cent and an A1203 content of about 3 per cent, based on the weight of the bath, willbe sufficient to give effective tanning. It is practicable and in some instances preferred to use a lesser amount of either constituent, but usually it will be desirable to keep the silicic acid content of the bath at a concentration of about 1 per cent SiOz or more, at least during the first part of the tanning procedure. It will usually be preferably-to have the silica and alumina compounds present in such proportions that the SiOztAlzOs ratio is about 2.:1.

The tanning compositions of this invention may be used to make up tanning baths for the treatment of any type of skinadapted to be converted to leather or fur. It is to be observed that the term skin is used generically to include heavy skins such as cowhide, lighter skins such as goatskin and calfskin, and also skins of fur-bearing animals. It will naturally be necessary to adapt the processes to the type of skin to be treated. For instance, goatskins may be more rapidly tanned than calfskins, but those skilled in the art are well aware of such factors and may readily adapt the compositions of this invention to the specific Skins to be tanned employing compositions of this invention may be prepared in any of the manners cutomary to the art. They may suitably b unhaired, limed, delimed, hated, and pickled. The skins may if desired be depickled. They may also be given a salt liquor treatment to soften them and to put them in a more uniform condition as is the practice in prior art tanning processes.

While skins prepared in any manner may be treated using tanning solutions made up with compositions of this invention, the pH of the skins is important. As will be observed below, the tanning is effected at certain conditions of acidity. and the skins may tend to change these conditions. The tanning solution may suitably be controlled to compensate for variance of the pH of the skins from that of the tanning solution. It may sometimes be found advantageous to adapt the processes for preparing the skins so that a skin prepared for tanning is approximatelyat the pH of the tanning solution.

It is desirable thattanning baths made up according to the present invention be used without prolonged storage since the baths as made up contain silicic acid having an average molecular,

weight sufiiciently low to give satisfactory tanning, but upon standing the molecular weight of the silicic acid increases, probably by polymerization, to the point where its penetration into skins and combination therewith is rendered difficult.

This difiiculty is alleviated to a considerable extent by the inclusion in the tanning bath of a hydrogen bonding donor compound,

Although the molecular weight of the silicic acid in the tanning solutions of this invention may vary depending on such factors as the manner of preparation of the solution and the age of the solution, by maintaining the pH at not higher than about 3.5 during the preparation of the solution the molecular weight of the silicic acid is held at a relatively low figure. The stability of the silicic acid toward precipitation with gelatin may be used as an indication of; the molecular weight, sols of high molecular weight being readily precipitated. According to this invention it has been found that the silicic acid should have a molecular weight such that when 2 cc. of a 1 per cent gelatin solution containing 4.2 per cent by weight of sodium sulfate (NazSO4) at a pH of 2.5 is added to a 5 cc. sample of the silicic acid solution adjusted immediately before the test to a pH of 2.5, an S102 content of 3 per cent by weight, and a sodium sulfate (NazSOr) content of 4.2 per cent by weight, no mdre than 0.4 cc. of diethyl ether of diethylene glycol is required to be added to the 5 cc. silicic acid sample to prevent precipitation of silicic acid.

Skins are tanned in accordance with a Process of this invention by immersing them in a tanning solution prepared as above described and agitating them in contact with such solution, as for instance in a tanning drum, at a pH of about 1.0 to 3.5, until penetration of the skins by the tanning agent has been effected. After satisfactory penetration has been obtained the pH of the solution is raised by the addition of any suittable alkali. Raising the pH accelerates the tanning action. The pH should at this point be raised to about 4.5 to 5.5. Sodium carbonate or other alkali metal or ammonium carbonates or hydroxides or organic bases such as ethanolamines may be used to raise the pH. Salts of organic acids which function as masking agents may also have some ability to raise the pH of the solution, especially from the pH 1.5 to 3.5 of the original solution. Thus, sodium acetate may be used to advantage for this purpose.

Consideration of the type of apparatus to use and the time to be allowed for tanning, the temperatures of tanning, and the amounts of tanning agent are substantially those known to the art in connection with prior art tanning processes. The time allowed for tanning must be suflicient to permit complete penetration of the tanning agent and the pH should not be raised too rapidly or there may be puckering and apparent over-tannage of the grain surface. The tanning can best be effected at room temperature, say 20 to 30 C. The exact amountof the tanning solution to use can best be determined by a few simple tests under the specific conditions of the process selected and with a specific skin'to be treated. In general, enough of the tanning agent must be used'to effect the tanning desired, but unduly large amounts shouldnot be used since they will not be taken up by the leather in a reasonable period of time and they may cause over-tannage;

In order that the invention may be better understood reference should be had to the following illustrative examples.

Example I 'A precipitated sodium aluminum silicate was prepared as follows:

Twenty-four and nine-tenths parts by weight of commercial iron-free aluminum sulfate (containing 16 per cent A1203) dissolved in 132 parts by weight of water was slowly added with vigorous agitation to a solution containing 31.5 parts byweight of a sodium silicate solution containing Na2O=9.1 per cent and SiO-2=29.7 per cent and 10.0 parts byweight of flake caustic (equivalent to 6.85 parts of NazO) dissolved in parts by weight of water. The SiO2:NazO ratio by weight of the final silicate solution used was 0.96. The additional sodium hydroxide was used therefore to adjust the alkalinity to that required to maintain a pH of about 9 during the reaction. The aluminum sulfate was added directly into the vortex of a stirrer so as to have rapid and efllcient mixing. After all the aluminum sulfate had been added, the reaction slurry was stirred for 15 minutes to insure complete reaction. This slurry contained 18.5 parts of sodium aluminum silicate and 16 parts of sodium sulfate'in approximately 330 parts by weight of water. Throughout the reaction the pH was above 9 and at the end of the reaction was about 10.5.

The product was filtered out, washed to remove the sodium sulfate and then dried for 15 hours at l10115 C.' 18.0 parts by weight of the dry product was obtained containing 43.20 per cent S102, 22.41 per cent A1203, 17.24 per cent NazO and 17.70 per cent ignition loss. The yield was 97.2 per cent of the theoretical. This particular product had a ratio of $102 to A1203 of 3.28 to 1, and a ratio of Na20 to A1203 of 1.26:1. From the composition of the material it was found that all the aluminum and 87-88 per cent of the silicate used in the reaction appear in the final product. The product was amorphous, soft, chalky, white, and opaque.

A tanning solution of this invention was made from this precipitated sodium aluminum silicate in the following manner:

Two hundred and four parts by weight of concentrated sulfuric acid was added to 4016 parts of water. To this solution was added 249 parts of the sodium aluminum silicate with vigorous agitation. During dissolution the pH did not rise above 2.5. After dissolution of the sodium aluminum silicate the pH of the solution was admasking agent.

justed to 22 by adding the necessary amount of sulfuric acid.

,A portion of the solution thus prepared containing'the equivalent of 3 parts by weight of SiOz was withdrawn and to it was'added 1.25 parts of sodium chloride. This solution was placed in a tanning drum and 50 parts, drained pickled weight, of goatsklns was added. These until the pHhad reached 5.5. The skins were then removed from the drum, washed, fatliquored, setout and crusted and finished according to conventional procedures.

The leather thus obtained was white in color and possessed excellent qualities such as'softness, fullness, and high shrink temperature, that is, ability to resist the action of hot water.

Example I!- According to this example, a tanning solution such as used in Example I was modified by adding to the portion used for tanning 3.2 parts by weight of diethyl ether of diethylene glycol, a hydrogen bonding donor compound. The test was carried out as described in Example I and the tanned skins had even better leather qualities than those of Example I. v n

Earample III To 100 partsby weight of the sodium aluminum silicate product obtained as in Example I there was added 90.7 parts by weight of commercial sodium acetate, NaC2H3O2-3H2O, to act as a centrated sulfuric acid to 4016 parts of water and adding 474.84 parts of the sodium aluminum'silicate sodium acetate mixture while maintaining the pH below about 2.5.by the addition of suitable amounts of. sulfuric acid. of the solution thus obtained a portion containing the equivalent of a parts by weight of $102 was withdrawn-and A tanning solution was then I made up by adding 204 parts by weight of con-- The pH. was then raised by v The leather thus obtained had excellent charsulfuric acid'to 1'70 parts of water. During this addition the acid solution was vigorously agitated and the pH of the solution was not allowed to rise above 2.2. To the silicic acid solution thus obtained there was added 36 parts by weight of commercial iron-free aluminum sulfate (containing 17 per cent A1203). The 'pH of the solution was again adjusted to 2.2 and there was added 10 parts of glacial acetic acid and- 5 parts of sodium chloride.

The solution thus obtained was used for tanning 200 parts, drained pickled weight, of goatskins which had previously been suitably prepared. The tanning was effected as in Example I by immersing the skins with agitation in the solution and gradually raising the pH. The finished leather obtained had'excellent qualities, was thoroughly and uniformly tanned, showing no surface hardness, was white, opaque, soft, and full. 1

Example V A tanning composition of this invention was made up by mixing 42.1 parts by weight of .commercial sodium meta-silicate, containing 28.5 per cent SiOz, and 13 parts of basic aluminum sulfate, containing 46.2 per cent A1203. To this mixture there was added 22.7 parts of sodium acetate to act as a masking agent.

A tanning solution was made up from this composition by adding the composition to a mixture of sulfuric acid in Water under such condidescribed in Example I by immersing the skins in the solution and raising the pH. The leather thus obtained was thoroughly and uniformly tanned and had generally excellent characterisdium glycollate.

acteristics of softness, fullness, and ability to resistwater at elevated temperatures and was completely tanned.

Example IV tics.

While in the foregoing. discussion there have been shown particular tanning compositions and solutions and particular tanning processes, it will. be understood that without departing from the scope of this invention one skilled in the art may employ-various compositions and processes.

I claim: I

v 1. A solid tanning composition comprising an acid-soluble sodium aluminum silicate and a masking agent selected from the group consisting of organic carboxylic acids having not more than eight carbon atoms and their salts.

2. A solid tanning composition comprising an acid-soluble sodium aluminum silicate and sodium acetate.

3. A solidtanning composition comprising an acid-soluble-sodium aluminum silicate and so- 4. A solid tanning composition comprising an acid-soluble sodium aluminum silicate and sodium lactate.

.5. A solid tanning composition comprising a masking agent selected from the group consist 

